The present application relates to a negative electrode for a lithium-ion secondary battery, the negative electrode containing a negative-electrode active material capable of occluding and releasing lithium ions, a lithium-ion secondary battery including the negative electrode, a battery pack, an electric vehicle, a power storage system, an electric tool, and an electronic device, which include the secondary battery.
In recent years, electronic devices typified by, for example, cellular phones and personal digital assistants (PDAs) have been widely used. Further size and weight reduction and longer life of electronic devices have been strongly demanded. Thus, there have been advances in the development of batteries serving as power sources, in particular, small and lightweight secondary batteries having a high energy density. Recently, various applications of secondary batteries to, for example, battery packs, electric vehicles, such as electric automobiles, power storage systems, such as power servers for household use, and electric tools, such as electric drills, as well as electronic devices described above have been studied.
Secondary batteries using various charge-discharge principles have been reported. In particular, lithium-ion secondary batteries that utilize the occlusion and release of lithium ions hold great promise because they have higher energy densities than lead-acid batteries, nickel-cadmium batteries, and other batteries.
A lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. The negative electrode contains a negative-electrode active material capable of occluding and releasing lithium ions. As the negative-electrode active material, a carbon material, such as graphite, is widely used. Recently, secondary batteries have been required to have higher battery capacities. Thus, the use of Si has been studied. The theoretical capacity of Si (4199 mAh/g) is much higher than the theoretical capacity of graphite (372 mAh/g), so the battery capacity should be significantly improved.
However, the use of Si as a negative-electrode active material causes extreme expansion and contraction of the negative-electrode active material during charge and discharge, so that the negative-electrode active material is liable to be cracked mainly in the vicinity of its surface. When the negative-electrode active material is cracked, a high-reactive newly-formed surface (an active surface) is formed, thereby increasing the surface area (reactive area) of the negative-electrode active material. As a result, the decomposition reaction of an electrolytic solution occurs on the newly-formed surface. The electrolytic solution is consumed to form a coating film derived from the electrolytic solution on the newly-formed surface. Thus, battery characteristics, such as cycle characteristics, are liable to decrease.
Thus, in order to improve battery characteristics, such as cycle characteristics, various configurations of the lithium-ion secondary batteries have been studied.
Specifically, to improve cycle characteristics and safety, Si and amorphous SiO2 are simultaneously deposited by a sputtering method (for example, see Japanese Unexamined Patent Application Publication No. 2001-185127). To obtain excellent battery capacity and safety performance, electron-conductive material layers (carbon material) are arranged on surfaces of SiOx particles (for example, see Japanese Unexamined Patent Application Publication No. 2002-042806). To improve high rate charge-discharge characteristics and cycle characteristics, a negative-electrode active material layer containing Si and O is formed in such a manner that the oxygen content is increased with decreasing distance from a negative-electrode collector (for example, see Japanese Unexamined Patent Application Publication No. 2006-164954). To improve cycle characteristics, a negative-electrode active material layer containing Si and O is formed in such a manner that the average oxygen content in the whole negative-electrode active material layer is 40 atomic percent or less and that the average oxygen content is increased with decreasing distance from a negative-electrode collector (for example, see Japanese Unexamined Patent Application Publication No. 2006-114454). In this case, a difference in average oxygen content between a portion near the negative-electrode collector and a portion remote from the negative-electrode collector is in the range of 4 atomic percent to 30 atomic percent.
To improve initial charge-discharge characteristics and the like, a nano-composite including a Si phase, SiO2, and metal oxide MyO is used (for example, see Japanese Unexamined Patent Application Publication No. 2009-070825). To improve cycle characteristics, powdered SiOx (0.8≦x≦1.5, particle size range: 1 μm to 50 μm) and a carbonaceous material are mixed and fired at 800° C. to 1600° C. for 3 hours to 12 hours (for example, see Japanese Unexamined Patent Application Publication No. 2008-282819). To shorten an initial charge time, a negative-electrode active material expressed as LiaSiOx (0.5≦a-x≦1.1 and 0.2≦x≦1.2) is used (for example, see International Publication No. WO2007/010922). In this case, Li is deposited by evaporation on an active material precursor containing Si and O. To improve charge-discharge cycle characteristics, the composition of SiOx is controlled in such a manner that the molar ratio of the O content to the Si content of a negative-electrode active material is in the range of 0.1 to 1.2 and that a difference between the maximum value and the minimum value of the molar ratio of the O content to the Si content in the vicinity of a boundary between the negative-electrode active material and a current collector is 0.4 or less (for example, see Japanese Unexamined Patent Application Publication No. 2008-251369). To improve load characteristics, a Li-containing porous metal oxide (LixSiO: 2.1≦x≦4) is used (for example, Japanese Unexamined Patent Application Publication No. 2008-177346).
To improve charge-discharge cycle characteristics, a hydrophobic layer of a silane compound, a siloxane compound, or the like is formed on a thin film containing Si (for example, see Japanese Unexamined Patent Application Publication No. 2007-234255). To improve cycle characteristics, a conductive powder in which surfaces of SiOx (0.5≦x<1.6) particles are covered with graphite coating films is used (for example, see Japanese Unexamined Patent Application Publication No. 2009-212074). In this case, on Raman spectroscopy analysis, each graphite coating film develops broad peaks at 1330 cm−1 and 1580 cm−1 Raman shift, and an intensity ratio I1330/I1580 is 1.5<I1330/I1580<3. To improve a battery capacity and cycle characteristics, a powder including 1% by mass to 30% by mass of particles is used, the particles each having a structure in which Si microcrystals (crystal size: 1 nm to 500 nm) are dispersed in SiO2 (for example, see Japanese Unexamined Patent Application Publication No. 2009-205950). In this case, in a particle size distribution by a laser diffraction/scattering type particle size distribution measurement method, the 90% accumulated diameter (D90) of the power is 50 μm or less, and the particle diameters of the particles are less than 2 μm. To improve cycle characteristics, SiOx (0.3≦x≦1.6) is used, and an electrode unit is pressurized at a pressure of 3 kgf/cm2 or more during charge and discharge (for example, see Japanese Unexamined Patent Application Publication No. 2009-076373). To improve overcharge characteristics, over-discharge characteristics, and the like, an oxide of silicon with a silicon-oxygen atomic ratio of 1:y (0<y<2) is used (for example, see Japanese Patent No. 2997741).
Furthermore, in order to electrochemically accumulate or release a large amount of lithium ions, an amorphous metal oxide is provided on surfaces of primary particles of Si or the like (for example, see Japanese Unexamined Patent Application Publication No. 2009-164104). The Gibbs free energy when the metal oxide is formed by oxidation of a metal is lower than the Gibbs free energy when Si or the like is oxidized. To achieve a high capacity, high efficiency, a high operating voltage, and long lifetime, it is reported that a negative-electrode material in which the oxidation numbers of Si atoms satisfy predetermined requirements is used (for example, see Japanese Unexamined Patent Application Publication No. 2005-183264). The negative-electrode material contains Si with an oxidation number of zero, a Si compound having a Si atom with an oxidation number of +4, and a lower oxide of Si having a silicon atoms with oxidation numbers of more than zero and less than +4.